lambda表达式，stream、filter、collect、set、limit、findFirst、Map、mapToInt、Distinct、sort、sorted、assertEquals
要使用lambda表达式，必须装1.8以上的Java版本（包括1.8）
一 stream（流）
1 通过filter过滤掉不满足条件的属性值（普通类型）
显示list集合中不是bb的所有值
public class FirstClass {
@Test
public void test() {
List list=new ArrayList();
list.add("aa");
list.add("bb");
list.add("cc");
list.add("dd");
list.add("ee");
list.stream().filter(s -> null!=s&&!"bb".equals(s))
.forEach(s -> System.out.println(s));
}
}
.filter(条件表达式，就是判断语句)
.forEach(要循环的值)，.forEach中的变量已经是过滤之后的了
2 通过.filter过滤引用类型，获取姓名不是aa对象的年龄
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
list.stream().filter(p ->!p.getName().equals("aa"))
.forEach(p->{ System.out.println(p.getAge()); } );
}
}
3 若是只想做一下过滤，不想输出，需要把过滤结果给其他人使用，怎么办？
.collect 是将.filter过滤完成的结果转为一个新的集合，需要用变量接收
import lombok.Data;
@Data
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
List list1=list.stream().filter(p ->!p.getName().equals("aa"))
.collect(Collectors.toList());
for(int i=0;i 
4 set集合也可以用.collect接收过滤后的结果
import lombok.Data;
@Data
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
Set set=new HashSet<>();
set.add(person);
set.add(person1);
set.add(person2);
Set set1=set.stream().filter(p ->!p.getName().equals("aa"))
.collect(Collectors.toSet());
Iterator iterator=set1.iterator();
while (iterator.hasNext()){
Person s=(Person) iterator.next();
System.out.println(s.getName());
}
}
}
5 其实特性
limit 例：List.stream().limit(2)返回前两个集合
import lombok.Data;
@Data
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
list.stream().limit(2).forEach(f->{
System.out.println(f);
});
}
}
6 count 例：List.stream().count()返回长度大小，与list.size()一个功能
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
long a=list.stream().count();
System.out.println(a);
}
}
7 findFirst 例：Optional p=list.stream().findFirst();返回集合中的第一个对象要用Optional接收，写入泛型类型
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
Optional p=list.stream().findFirst();
System.out.println(p);
}
}
8 Map就是将某个List内容转换成Map
例把两个人的年龄放到一个新的List里，map里的语法叫函数式接口
语法（引用类型::方法名）,就是遍历引用类型Person类，调用getName方法，将获取到的name放在一个新的集合list1里
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
List list1=list.stream().map(Person::getName).collect(Collectors.toList());
for(int i=0;i 
9 将List集合里的对象年龄相加，mapToInt(Person::getAge)，还可以mapToDouble还可mapToIong
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("bb");
person1.setAge(11);
Person person2 = new Person();
person2.setName("cc");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
int ageSum=list.stream().mapToInt(Person::getAge).sum();
System.out.println(ageSum);
}
}
map里要是一般值，不是对象怎么取值，直接get（“Key”）就好
@org.testng.annotations.Test
public void mapTest(){
List> mapList=new ArrayList<>();
Map map =new HashMap<>();
map.put("userName","aa" );
map.put("age","十岁" );
map.put("sex","女" );
Map map1 =new HashMap<>();
map1.put("userName","bb" );
map1.put("age","九岁" );
map1.put("sex","男" );
mapList.add(map);
mapList.add(map1);
List> mapList1=mapList.stream().
filter(m ->m.get("userName").
equalsIgnoreCase("bb")).
collect(Collectors.toList());
for (Map m:mapList1){
System.out.println(m);
}
}
10 去重
Distinct:将多个相同内容只保留一个，与set相似，相同元素，只保留一个；据说对象去重要重写HashCode,但是下面这段代码并没有重写HashCode也实现了对象去重
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("aa");
person.setAge(10);
Person person1 = new Person();
person1.setName("aa");
person1.setAge(10);
Person person2 = new Person();
person2.setName("aa");
person2.setAge(10);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
list.stream().distinct().forEach(p->{
System.out.println(p);
} );
}
}
11 对象排序方法1
对象排序要指定一个属性，因为对象中有很多个属性（例如：name、age、sex…），并不知道要用哪个属性排序
(Person p1,Person p2)指定的属性p1和p2是两两对比，调用的就是排序方法，排序用的是ASCII码表顺序作为排序依据
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("bb");
person.setAge(10);
Person person1 = new Person();
person1.setName("zz");
person1.setAge(11);
Person person2 = new Person();
person2.setName("xx");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
list.sort((Person p1,Person p2) ->p1.getName().compareTo(p2.getName()));
list.forEach(p-> {
System.out.println(p.getName());
});
}
}
**12 对象排序方法2（返序）
list.stream().sorted(Comparator.comparing(Person::getName));
list.forEach(p-> {
System.out.println(p.getName());
});
输出的还是bb zz xx 那就是没有排，name用.stream()的方式干什么呢？返序
在.sorted()后面加上.reversed()做返序排序；返序完成之后是一个新的流，要用一个变量接收（看清楚，是List list 是Person不是String），才能用forEach打印，或者在当前流直接forEach**
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("bb");
person.setAge(10);
Person person1 = new Person();
person1.setName("zz");
person1.setAge(11);
Person person2 = new Person();
person2.setName("xx");
person2.setAge(12);
List list=new ArrayList();
list.add(person);
list.add(person1);
list.add(person2);
list.stream().sorted(Comparator.comparing(Person::getName).reversed()).forEach(s ->{
System.out.println(s.getName());
});
}
}
13 如何将对象存入map里，从map里取出？
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("bb");
person.setAge(10);
Person person1 = new Person();
person1.setName("zz");
person1.setAge(11);
Person person2 = new Person();
person2.setName("xx");
person2.setAge(12);
Map map=new HashMap<>();
map.put("AA",person );
map.put("BB",person1 );
map.put("CC",person2 );
Person p=map.get("AA");
System.out.println(p.getName());
}
}
14 如果我们不知道key的时候要获取name=bb这个对象的age，要怎么做？
对于map而言有一个约定，key用字符串存储，在lambda操作map里的值用数组保存
public class Person {
private int age;
private String name;
}
import org.junit.Test;
import java.util.*;
import java.util.stream.Collectors;
public class FirstClass {
@Test
public void test() {
Person person = new Person();
person.setName("bb");
person.setAge(10);
Person person1 = new Person();
person1.setName("zz");
person1.setAge(11);
Person person2 = new Person();
person2.setName("xx");
person2.setAge(12);
Map map=new HashMap<>();
map.put("AA",person );
map.put("BB",person1 );
map.put("CC",person2 );
map.forEach((k,v) ->{
if(v.getName().equals("bb")){
System.out.println(v.getAge());
}
});
}
}
15 在lambda表达式中操作Map里对象的值用数组保存
有3个老师，每个老师有2个属性：name、course，每个老师的姓名，所授课程不同，根据输入的课程判断该课程对应的老师是否与预期相同
1.Teacher 类
import lombok.Data;
@Data
public class Teacher {
String name;
String coures;
}
老师1继承Teacher 类
public class Xiaohu extends Teacher {
public Xiaohu(String name,String coures){
this.name=name;
this.coures=coures;
}
}
老师2继承Teacher类
public class Xiaoli extends Teacher {
public Xiaoli(String name,String coures){
this.name=name;
this.coures=coures;
}
}
老师3继承Teacher类
public class Xiaowanzi extends Teacher{
public Xiaowanzi(String name,String coures){
this.name=name;
this.coures=coures;
}
}
业务类：实例化老师，做逻辑判断，获取的teacher的名字用数组接收，要先定义一个数组
import org.junit.Test;
import java.util.HashMap;
import java.util.Map;
import static org.junit.Assert.*;
public class TeacherTest {
public String teacherTest(String coures){
Xiaoli xiaoli=new Xiaoli("xiaoli","java");
Xiaohu xiaohu=new Xiaohu("xiaohu","APP");
Xiaowanzi xiaowanzi=new Xiaowanzi("xiaowanzi","seleium");
Map teacherMap =new HashMap<>();
teacherMap.put("xiaoli", xiaoli);
teacherMap.put("xiaohu",xiaohu);
teacherMap.put("xiaowanzi", xiaowanzi);
String [] name=new String[1];
teacherMap.forEach((k,v) ->{
if(v.getCoures().equals(coures)){
name[0]=v.getName(); }});
return name[0];
}
测试类，做检查点
@Test
public void test(){
String name=teacherTest("java");
assertEquals("校验老师名字","xiaoli" ,name);
}
}
assertEquals引import static org.junit.Assert.*;包，别引错包，第一个参数是输出文案，第二个参数是预期结果，第三个参数是实际结果